1. Field of the Invention
This invention relates to a method of making ceramic articles. More particularly, it relates to a method of making ceramic articles by use of an organic binder which can provide good granulation properties in forming ceramic powder into granules and, when such granules are shaped, can produce shaped bodies having high green (or unfired) strength and, moreover, dense fired products.
2. Description of the Prior Art
Ceramic articles made by firing powder materials such as alumina, ferrite, tungsten carbide, silicon nitride and silicon carbide have come to be used in a wide range of applications including electrical insulating materials, magnetic materials, electronic components, mechanical parts, automobile parts and cutting tools.
Conventionally, such ceramic articles are generally made as follows: First, a slurry is prepared by mixing a ceramic powder material with a solvent or water and an organic binder. Then, using a spray dryer, the slurry is formed into granules. Alternatively, an organic binder dissolved in a solvent or water is added to a ceramic powder material and they are blended by means of an automatic mortar or the like. The resulting agglomerates are crushed and adjusted to a desired size. The granules thus obtained are shaped by means of, for example, a compression molding machine, rubber pressing machine or hot pressing machine. Alternatively, the granules can also be shaped by extrusion molding or slurry casting. Then, the resulting shaped bodies are fired to produce a variety of ceramic articles.
If the unfired, shaped bodies have low green strength, they are often cracked or chipped during the succeeding binder removal and firing steps. Accordingly, the unfired, shaped bodies must have sufficiently high green strength in order to achieve a reduction in rejection rate and hence an improvement in productivity. On the other hand, in order to prevent the die from being damaged and in order to produce dense shaped bodies containing few pores, it is necessary that the granules used as the starting material be easily collapsed and shaped under moderate pressure during the shaping step. To this end, it is desirable to use an organic binder having wellbalanced performance characteristics. An additional property required for organic binders is good thermal decomposability. An organic binder having poor thermal decomposability requires high temperatures and long periods of time in the binder removal step. In some cases, residual carbon may remain in the final ceramic product, resulting in a significant reduction in electrical properties, magnetic properties and physical or mechanical properties.
Typical organic binders which have been used in the prior art include, for example, polyvinyl alcohol, alkali metal salts of carboxymethyl cellulose and alkali metal salts of acrylic resins. However, these binders have been disadvantageous in many respects. Specifically, polyvinyl alcohol has been commonly used because of its low price, but it may gather mold during storage and may give off harmful gases and a bad odor to create an undesirable working environment. Moreover, the granules formed with polyvinyl alcohol are so hard that considerable damage is caused to the shaping die. In addition, the granules are difficult to collapse and, therefore, produce shaped bodies which contain many pores and tend to become cracked and/or chipped. Furthermore, polyvinyl alcohol exhibits poor thermal decomposability during the firing step, so that the presence of much residual carbon prevents the production of dense ceramic articles. On the other hand, alkali metal salts of carboxymethyl cellulose give high viscosity even when used in low concentrations, and are difficult to handle because of their thixotropic properties. Moreover, like alkali metal salts of acrylic resins, they exhibit poor thermal decomposability. Thus, they also fail to produce dense ceramic articles, similar to polyvinyl alcohol.